Optimize SFTP download performance

[VulcanoSoftware]

Optimized SFTP download speed by:

1. Spawning concurrent background producers in MessageApi.download_file_parallel to achieve true multi-connection parallel downloads across HTTP Range sub-requests.
2. Improving DCFSSFTPBufferedFile read buffering in the SFTP handler to support forward skips up to 2MB and retain recently fetched data, preventing stream cancellations and re-downloads during SFTP client pipelining and out-of-order read requests.
3. Adding unit tests for parallel message downloads and SFTP pipelined reads.

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PR created automatically by Jules for task 14659272526158921512 started by @VulcanoSoftware

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